Group A Streptococcus (GAS, Streptococcus pyogenes) is a leading bacterial pathogen of the human pharynx and skin. In most cases the pathogen causes self-limiting infections such as pharyngitis and impetigo. However, in recent years, a striking resurgence in severe invasive GAS infections has been observed worldwide. GAS infections account for more than 650,000 cases of invasive disease annually. Although this pathogen is susceptible to antibiotic therapy, severe invasive GAS infections are often difficult to treat. The ability of GAS to cause infections is associated with pathogen resistance to killing by the host innate immune system. The cell wall of GAS consists of peptidoglycan sacculus decorated with a carbohydrate comprising a polyrhamnose backbone with immunodominant N-acetylglucosamine side-chains, the Group A Carbohydrate (GAC). GAC plays a major role in the bacterial physiology of GAS by protecting the cell envelope from environment stress and innate immunity assault. To evade immune recognition, GAS deploys post-synthetic modifications of GAC sugars. The goal of this proposal is to re-examine GAC structure, and characterize GAC biosynthesis, regulation and function. To help answer these important biological questions we will use a range of biochemical and structural approaches, including NMR, x-ray crystallography and atomic force microscopy. The biological function of GAC sugar modifications will be studied in different models of GAS infection. The proposed study is significant because GAC is a promising candidate for a universal GAS vaccine and the enzymes of the GAC biosynthesis pathway are attractive drug targets.